“…Motivation for Fentanyl Following acquisition, in a subset of OVX rats (Fig. 1a; OVX+E, N = 17 and OVX+V, N = 14), a baseline level of motivation for fentanyl was established prior to IntA fentanyl self-administration (pre-IntA PR) using a progressive-ratio schedule as previously described [38]. Briefly, the response requirement to obtain a fentanyl infusion increased throughout the session in the following steps: 1, 2, 4, 6, 9, 12, 15, 20, 25, 32, 40, 50, 62, 77, 95, 118, 145, 178, 219, 268, 328, 402, etc.…”